1. Field of the Invention
The invention relates to a channel encoder for encoding received n-bits information words to m-bit, mainly DC-free, codewords, where m&gt;n. The channel encoder comprises a memory circuit having an input connected to a counter. The memory circuit is provided for receiving each time the n-bit information word and the counter is provided for determining the digital sum variation (DSV) between two successive received binary words.
2. Prior Art
Channel encoders are widely used in digital magnetic recording and playback apparatus in order to adapt the inputted data stream to the characteristics of the data transmission channels. In such apparatus the flux is differentiated in time by the read head. Ignoring some high-frequency losses, this flux has the same shape as the write current applied to the write head. The output signal of the read head then consists of positive and negative pulses at the areas where transitions occur in the write current. In order to obtain a replica of the write current at the playback side in said apparatus, so-called pulse amplitude detection methods can be used in addition to the detection by so-called "write current restoration", that is to say by integration of the output signal of the read head. In the pulse amplitude detection methods the pulses in the output signal of the read head are detected. Even though the signal reproduced is always DC-free as a result of the differentiation, for the latter detection methods preferably the write current is also mainly DC-free, because otherwise problems could arise when (rotating) transformers are arranged between the write amplifier and the write head or when other signals (tracking tones) must alsio be recorded at low frequencies.
A channel encoder of this kind is known from the French patent application No. 2,469,047. In the known channel encoder the n-bit information word is supplied to the memory circuit as well as to the counter. The counter determines each time the digital sum variation between two successively supplied information words. The digital sum variation is determined in order to form the m-bits codeword and to provide that the coded signal thus formed is mainly DC-free. Therefore if the digital sum variation deviates too much from zero the supplied information word is inverted and the appended (m-n) word indicates that inversion has occurred.
A drawback of the known encoder is that by using the n-bit information word for determining the digital sum variation it is not possible to use a precoder while maintaining the encoded signal mainly DC-free. Indeed if the encoded signal as provided by the known encoder should be supplied to a precoder, the outputted signal would no longer be DC-free.